All content in the level is lost during the migration project, and a folder named __ExternalActors__ exists in the Content folder

The version I used was 4.27, and when I put an actor in a sequence in the scene, that actor was shown red in the track, and when I migrated the level, none of the resources in the level were identified as being associated with the level, and when I looked at the reference graph, The level does not reference any resources other than sublevels. After looking at the Content folder, I noticed that there was an ExternalActors folder that contained some of the resources in the level. Does anyone know how to solve this problem´╝č